Natural gas is transported from place-to-place via pipelines. It is desirable to know with accuracy the amount of gas flowing in the pipeline, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Even where custody transfer is not taking place, however, measurement accuracy is desirable, and in these situations flow meters may be used.
Ultrasonic flow meters are one type of flow meter that may be used to measure the amount of fluid flowing in a pipeline. Ultrasonic flow meters have sufficient accuracy to be used in custody transfer. In an ultrasonic flow meter, acoustic signals are sent back and forth across the fluid stream to be measured. Based on parameters of received acoustic signals, the fluid flow velocity in the flow meter is determined. The volume of fluid flowing the meter can be determined from determined flow velocities and the known cross-sectional area of the flow meter.
The transit time of acoustic signals in an ultrasonic flow meter includes time required for signals to travel through fluid flowing in the meter, time that the acoustic signals spend within the transducers that produce and detect the signal, and time required to process the signals. In order to accurately determine fluid flow velocity, and thus to determine the value of the fluid, each of the aforementioned components of the signal transit time must be accurately determined.